The operating capabilities of short range communication radio sets can be materially improved by use of repeaters. The effective range of radio sets is often reduced by natural obstacles such as dense foliage or hilly terrain. A strategically placed repeater may be used to overcome these natural obstacles by providing a transmission path free of obstructions, thereby extending the effective range of the radio set and improving its tactical value. For example, troops often set up radio sets in valleys or behind hills for protection and a hilltop repeater in such a situation can greatly expand the capability of such tactical radio sets.
Typically, repeaters operate either by receiving and retransmitting at the same frequency, repeaters of this type being known as F1-F1 repeaters, or by receiving at one frequency and, retransmitting at a second frequency, repeaters of this type being known as F1-F2 repeaters.
F1-F1 repeaters are more desirable in many applications since they utilize less hardware and require a smaller portion of the frequency spectrum. However, in these repeaters some means must be provided to prevent coupling between the transmitter and receiver, which can cause oscillation or singing.
The present invention is a repeater in which signal amplification is achieved by means of an injection locked oscillator. The incoming signal, which may be too noisy for direct amplification, is applied to an oscillator with a free-running frequency substantially equal to the center or carrier frequency of the incoming frequency modulated signal. The oscillator locks onto the injected signal and follows the modulation thereon. The oscillator output is then amplified and transmitted. The FIGURE of merit of such an injection locked oscillator is the locking gain, defined as the ratio of power output to power input. The locking gain is inversely proportional to the difference between the free-running or center frequency of the oscillator, and the instantaneous injection frequency. Thus, as the deviation of the received injected signal increases, the locking gain decreases. The present invention involves a means for permitting the free-running or center frequency of the oscillator to follow or track the frequency deviations of the injected signal, so that the average frequency deviation is greatly reduced and hence the locking gain stabilized and maximized.